The present invention relates generally to test and measuring equipment, and more particularly, to a single cable, single point, stimulus and response probing system and method for use in test and measuring equipment.
Circuit testing is an indispensable part of circuit design, circuit analysis, and circuit troubleshooting. Many devices are available which perform various testing functions. For example, digital multimeters, oscilloscopes, and logic analyzers are a few of the devices in use that perform various circuit testing functions, including but not limited to voltmeter functions such as voltage, resistance, and continuity; component testing functions such as diode operation, capacitance, and inductance; analyzing a logic circuit; pulse sine and arbitrary waveform generation; and frequency counting. Typically, circuit testing involves providing a stimulus to a circuit and then measuring a response. Voltmeter functions and component testing are examples of stimulus measurements, while logic analyzer and oscilloscope measurements are examples of response measurements.
Typically, a coaxial cable connects a device under test (DUT) to the stimulus and response test and measurement device. The DUT can be any circuit, component, or logic device to be analyzed or tested. Examples are discrete components such as resistors, capacitors, diodes; integrated circuit components including application specific integrated circuits (ASICs); and logic circuitry. The aforementioned single cable arrangement presents a large capacitive load (typically greater than 100 pF) to the DUT, limiting measurement to a restricted bandwidth. For higher bandwidth measurements, two coaxial cables have typically been required. One cable connects the stimulus source to the DUT, while a separate cable connects the DUT to the response measurement circuitry. This arrangement allows the measurement of higher bandwidth signals, but requires the user to manage multiple sets of test cables.
Typically, in higher bandwidth applications, the stimulus output is low impedance (50 or 75 ohms typical), and therefore, a low impedance connection is required between the stimulus source and the DUT. In order to achieve higher bandwidth response measurements, the connection between the DUT and the response measurement circuitry is high impedance (typically 100K-10M ohms,  less than 20 pF), so as to prevent or minimize distortion of the signal being measured. Loading the DUT this way is generally acceptable for measuring frequencies up to approximately 500 MHz. This arrangement typically places in the tip of the probe cable components that electrically isolate the capacitive loading of the cable from the DUT. However, these components in the probe tip generally add impedance between the DUT and the test equipment, making this cable unusable for connecting stimulus circuitry to the DUT,
Therefore, it would be desirable to provide a single test cable capable of allowing both low impedance stimulus measurement and high impedance response measurements in high bandwidth applications.
The present invention provides an improved probing system and method used with test and measurement equipment. The single cable, single point, stimulus and response probing system disclosed herein is applicable to hand held test equipment and to any test or measurement device that measures both high and low impedance circuits.
The present invention is a probing system and method capable of both low impedance stimulus measurements and high impedance response measurements, the probing system comprising a cable, stimulus circuitry configured to supply a first stimulus signal to the cable, and response circuitry configured to receive an output of the cable. The cable is essentially two signal wires and a ground shield. In a first embodiment, the cable includes a first diode and a resistor at or near the probe tip.
A second embodiment of the probing system adds an additional diode and signal wire to the cable, and adds multiple embodiments of the stimulus circuitry. A first embodiment of the stimulus circuitry is configured to deliver a first stimulus signal to both diodes located within the probe. As in the first embodiment, response circuitry is configured to receive an output of the cable.
A second embodiment of the stimulus circuitry adds a push-pull amplifier circuit and two additional diodes to the stimulus circuitry, the additional circuitry configured to deliver to the first amplifier a local feedback signal. The local feedback signal is taken from the output of the stimulus circuitry.
A third embodiment of the stimulus circuitry includes the push-pull amplifier of the previous embodiment and adds a second amplifier, located within the response circuitry. The second amplifier is configured to receive as input a signal from the probe, buffer that signal, and deliver it as loop feedback to the first amplifier located within the stimulus circuitry. The loop feedback signal is representative of the voltage signal located at the probe tip, but is buffered by the second amplifier before delivery to the first amplifier.
A fourth embodiment of the stimulus circuit eliminates the push-pull amplifier circuitry and relies on the output of the first amplifier to draw sufficient current to drive the diodes located in the probe.
The present invention can also be conceptualized as providing a method for providing stimulus to a single cable probing system, the probing system capable of both low impedance stimulus measurements and high impedance response measurements, comprising the following steps. First, a stimulus signal is supplied to a probe cable, the probe cable including a first diode and a resistor. Next, a response signal is received in response circuitry, the response signal being supplied from the probe cable. In a second embodiment, a second diode is added to the probe cable.
The invention has numerous advantages, a few of which are delineated hereafter, as merely examples.
An advantage of the single cable, single point, stimulus and response probing system and method is that it provides the ability to perform both low impedance stimulus measurements and high impedance response measurements using a single probe cable.
Another advantage of the single cable, single point, stimulus and response probing system and method is that it by using a single cable it reduces the complexity and amount of time required to perform various circuit and component test and measurement functions.
Another advantage of the single cable, single point, stimulus and response probing system and method is that it permits the testing and measurement of many passive devices and active circuit elements.
Another advantage of the single cable, single point, stimulus and response is probing system and method is that it permits the testing and measurement of circuits.
Another advantage of the single cable, single point, stimulus and response probing system and method is that it is simple in design, reliable in operation, and its design lends itself to economical mass production.
Another advantage of the single cable, single point, stimulus and response probing system and method is that it lends itself to implementation on an application specific integrated circuit (ASIC) for economical mass production.
Other features and advantages of the present invention will become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional features and advantages be included herein within the scope of the present invention, as defined in the appended claims.